1. Field of the Invention
The present invention relates to preventing the contamination of a wafer in ion implantation equipment, and more particularly, to preventing the periphery of the wafer from being contaminated by metallic ions generated during an ion implantation process.
2. Description of the Related Art
In general, ion implantation in the manufacture of semiconductor devices refers to technology that injects ions into a given target. The ion implantation equipment supplies energy to the ions sufficient to accelerate the ions to such a degree that they can penetrate the target surface.
In conventional ion implantation equipment, the concentration of impurities is controlled within a limit ranging from 10.sup.14 to 10.sup.18 atoms/cm.sup.3. Such equipment is widely used for implanting ions into a given target material, because it controls the concentration of impurities better than other impurity implantation techniques.
Ion implantation equipment generally includes at least the following components: vacuum loadlock, ion source, beam extracting and accelerating device, mass analyzer, accelerator tube, and a disk for mounting a target thereon. The equipment is designed to produce varying levels of high voltage to effect ion decomposition, extraction, and acceleration. During the ion implantation process the ion gas supplied from the ion source is turned into plasma and the ions are accelerated and extracted by an electric field formed by the applied voltage. The extracted ions form an ion beam. The amount and size of the ions are analyzed at a point where the ion beam is diffracted, and the ion beam is then accelerated sufficiently to penetrate the target to an intended depth.
The above-described conventional ion implantation equipment generally has a structure as shown in FIG. 1, and comprises an ion source 12, an ion extracting and accelerating device 14, a mass analyzer 16, an accelerator tube 18, a tandem electron chamber 20, and a disk 22 disposed in a path along which the generated ion beam propagates toward the wafer 10. In addition, a Faraday cup assembly 24 is provided to neutralize the environment in which the ions are injected into the wafer 10, and to measure the doping dose.
The ion source 12 supplies the source gas to the ion extracting and accelerating device 14. Positive ions are generated and extracted by the beam extracting and accelerating device 14 so as to form an ion beam 17, and the resultant ion beam 17 is diffracted and analyzed in the mass analyzer 16.
The ion beam 17 is accelerated in the accelerator tube 18 and tandem electron chamber 20, and then injected into the wafer 10 held on the disk 22. The amount of the ions injected into the wafer 10 is measured by the current meter 28 of the Faraday cup assembly 24.
When the ion beam 17 finally reaches the wafer 10 and the disk 22, the positive ions collide therewith beneath the wafer surface. The injected positive ions act as impurities inside the wafer 10. The reverse bias power source 26, integrated with the Faraday cup assembly, is used to constrain the movement of secondary electrons given off the target wafer when irradiated with the ion beam 17.
When the positive ions collide with one another in the disk 22, metallic ions are scattered from the disk 22. As a result, the wafer periphery is contaminated by the metallic ions as shown by the arrows in FIG. 1. The contaminants are mostly heavy metals, such as iron and aluminum. The contaminants at the wafer periphery adversely affect the semiconductor manufacturing process, thereby reducing the production yield.
The above problem occurs more often in ion implantation equipment wherein the ion implantation is conducted at an energy level higher than 300 keV, namely in connection with the so-called Deep Process.
The amount of the heavy metal contaminating the wafer periphery is proportional to the accelerating energy of the ion beam and the beam current. Accordingly, it is a generally accepted practice to reduce the accelerating energy of the ion beam and the beam current in order to prevent the heavy metal contamination of the wafer when using conventional ion implantation equipment.
However, because the accelerating energy of the ion beam is an important factor in the Projected Range (PR) or depth of implantation, there is a limit to how much the accelerating energy can be reduced in order to, in turn, limit the metal contamination.
Although a reduction of the beam current reduces the amount of the contaminants, this results in the reduction of the number of wafer-injected ions, and an increase in the time necessary to process a wafer. Accordingly, reducing the beam current results in a corresponding decrease in the manufacturing productivity.